Examples of heat exchangers of an automobile air conditioning system generally include a condenser which heat exchanges high temperature and pressure refrigerant with the ambient air to convert refrigerant into liquid state and an evaporator which transforms liquid refrigerant into low temperature gaseous phase to cool the indoor air.
Each of the condenser and the evaporator includes tubes having refrigerant passages through which refrigerant flows, corrugated heat radiating fins interposed between the tubes, header tanks connected with both ends of the tubes in a communicating fashion and inlet and outlet pipes installed in the header tanks for allowing refrigerant to flow into/out of the header tanks.
As an example of such a heat exchanger, the condenser adopts a flat tube having multiple passages as disclosed in Japanese Patent Publication No. 1999-159985.
Since the tube passages disclosed in the above document are elongated in a lateral direction of the tubes, in the event of reducing diameter to increase the number of passages, the thickness of upper and lower walls is increased thereby unnecessarily enlarging mass. Further, in case of reducing the hydraulic diameter of the passages in order to raise heat exchange efficiency in the tube of the same size, the thickness of the outside wall of the tube is unnecessarily increases.
In the prior art, passage junctions of the tube are provided in upper and lower sides of the tube passages so that an excessive quantity of condensate within a passage may fill a lower portion of the passage to degrade the effect of breaking a condensate film thereby deteriorating overall heat transfer performance.
Furthermore, a sharp leading end is formed in the passage of the tube, the leading end may be easily fractured or poorly shaped owing to the shape of a tool and limited endurance, thereby degrading productivity and product quality.